I'd like to know, will class transformation at classloader level be available in future releases of Tomcat ? I currently need to replace tomcat's webapp classloader by a custom one to be able to use JPA persistence with Tomcat (this isn't needed with Glassfish, for example).

Jakarta Tomcat's default classloader does not support class transformation but allows custom classloaders to be used. Spring offers the TomcatInstrumentableClassLoader which extends the Tomcat classloader (WebappClassLoader) and allows JPA ClassTransformer instances to 'enhance' all classes loaded by it. In short, JPA transformers will be applied only inside a specific web application (which uses the TomcatInstrumentableClassLoader).